Sodium hydroxide and chlorine which are important as industrial raw materials are mainly produced by brine electrolysis. This electrolytic process has shifted towards an ion-exchange membrane process using an ion-exchange membrane as a diaphragm and an active cathode having a low overvoltage, through a mercury process using a mercury cathode and a diaphragm process using an asbestos diaphragm and a soft iron cathode. This improvement decreased the electric power consumption rate for producing 1 ton of caustic soda to 2,000 kWh.
In the brine electrolysis using the active cathode, which has been most generally performed at present, the cathode is disposed in contact with or with a gap of 3 mm or less to a cathode side of the cation-exchange membrane. Water reacts at a catalytic layer to produce sodium hydroxide. An anodic reaction and a cathodic reaction are each as follows, and the theoretical decomposition voltage becomes 2.19 V.2Cl—=Cl2+2e− (1.36 V)2H2O+2e−=2OH—+H2 (−0.83 V)
DSA used as an anode has been operationally proven up to 200-300 A/dm2 in the mercury process. However, as the cathode in the ion-exchange process, it is important to have a low overvoltage, not to damage the membrane upon contact therewith and to provide less contamination due to metal ions and the like from the cathode. The proven active cathodes include an active electrode obtained by dispersing ruthenium powder in a Ni plating bath and performing composite plating using the resulting dispersion, a composite catalyst electrode comprising ruthenium oxide and nickel oxide, a Ni-plated electrode containing a second component such as S or Sn, a NiO plasma-sprayed electrode, a Raney nickel electrode, a Ni—Mo alloy electrode, a Pt—Ru immersion-plated electrode and an electrode using a hydrogen storage alloy for imparting resistance to reverse current. As reference documents, there are Electrochemical Hydrogen Technologies, pp. 15-62, 1990, U.S. Pat. No. 4,801,368, J. Electrochem. Soc. 137, 1419 (1993), Modern Chlor-Alkali Technology, vol. 3, 1986, and the like.
In recent ion-exchange membrane electrolysis technology, electrolysis cells which can increase the current density are being devised in order to increase production capacity and to decrease investment cost. Further, loading of high current has become possible by development of low-resistant membranes. In this case, it is desirable to dispose the cathode in close contact with (with a zero gap to) the ion-exchange membrane, because the voltage can be decreased. However, a conventional surface-roughened cathode is likely to mechanically damage the membrane, which has been a problem.
In order to solve this problem, cathodes using noble metals having high activity although having smooth surfaces have attracted attention. Such cathodes are disclosed in the following documents:    Patent Document 1: JP-A-2006-104502    Patent Document 2: JP-A-2006-193768    Patent Document 3: JP-A-2003-277966    Patent Document 4: JP-A-2003-277967    Patent Document 5: JP-A-2000-239882    Patent Document 6: JP-A-2006-299395    Patent Document 7: JP-A-2006-118022    Patent Document 8: JP-A-2006-118023    Patent Document 9: JP-A-2003-268584    Patent Document 10: JP-A-7-90664
The foregoing patent documents disclose the following, respectively:
Patent Document 1: A cathode for electrolysis comprising: a conductive substrate; an intermediate layer containing a conductive oxide; and a catalytic layer containing at least one member selected from silver and silver oxide, and at least one member selected from platinum group metals, platinum group metal oxides and platinum group metal hydroxides.
Patent Document 2: A cathode for generation of hydrogen comprising a cathode substrate and a catalytic layer formed thereon, wherein the catalytic layer contains cerium, platinum and ruthenium.
Patent Documents 3 and 4: A cathode for generation of hydrogen obtained by applying an aqueous solution containing a platinum group compound (preferably a ruthenium compound) and at least one member selected from lanthanum, cerium and yttrium compounds, onto a conductive substrate, followed by burning in the air to thermally decompose the aqueous solution, thereby forming a catalytic layer on the substrate.
Patent Document 5: A cathode improved in adhesion in which an intermediate layer containing a nickel oxide as a main component is provided between a substrate and a catalytic layer.
Patent Document 6: An electrode for generation of hydrogen comprising at least one member selected from the group consisting of platinum group compounds, lanthanum compounds, cerium compounds and yttrium compounds, niobium compounds and manganese compounds.
Patent Documents 7 and 8: An electrode for generation of hydrogen in which a platinum alloy comprising one metal selected from the group consisting of nickel, cobalt, copper, silver and iron and platinum is carried on a conductive substrate, and the amount of platinum contained in the platinum alloy is within the range of 0.40 to 0.99 in molar ratio.
Patent Document 9: A catalyst containing a ruthenium compound on a nickel substrate.
Patent Document 10: A low hydrogen overvoltage cathode obtained by performing electroplating in a nickel plating bath in which active carbon particles carrying a platinum group metal are dispersed to form an electrode active layer on an electrode substrate, the electrode active layer comprising nickel which contains the active carbon particles carrying the platinum group metal and has the particles adhered to a surface layer thereof.
There is a report that discharge of water (hydrogen ions) to hydrogen atoms proceeds on a main catalyst in an electrode for electrochemistry, a part of hydrogen atoms migrate to a hydrogen adsorption layer by spillover. For example, it is reported in non-patent document 1 that a spillover phenomenon of hydrogen occurs in Pt—C (a carbon substrate covered with platinum). Further, in non-patent document 2, it is reported that adsorption current of hydrogen increases in a Pt—TiO2 electrode.
However, there is no report that these materials have been utilized in order to generate hydrogen by hydrolysis of water. In other words, utilization of hydrogen adsorption for improving the hydrogen generation efficiency on a cathode for generation of hydrogen has not been known.    Non-Patent Document 1: Electrochemica. Acta, vol. 118, 473 (1973)    Non-Patent Document 2: Russian J. Electrochem., vol. 132, 1298 (1996)